The present invention relates to the preparation of cylinders for use in printing. It has particular relevance to the engraving of anilox rollers but is also applicable to other processes involving the engraving or exposure of cells on a rotating medium.
In conventional systems for preparing anilox cylinders, a ceramic-coated roller is rotated and a laser engraving head is moved along the roller parallel to its axis. The engraving head may, for example, include a plane mirror and lens which focuses light from a laser onto the surface of the roller. The laser is pulsed and the combined effect of the rotation of the roller and the lateral movement of the engraving head is to produce a helical pattern of cells with one cell corresponding to each pulse from the laser. By appropriate synchronisation of roller rotation, the speed of the engraving head, and the laser pulsing, the cells can be arranged in hexagonal, square or other patterns.
Whilst conventional engraving techniques, such as that described above, are generally satisfactory at lower speeds, a number of problems arise as the rate at which the cells are engraved increases. When engraving separated cells, for example, it is found that at high rotational speeds each cell tends to be elongated in the direction of rotation of the roller, producing a cell which is oval in form. When engraving contiguous cells at high speeds the wall between adjacent cells tends to break down, an effect known as "channelling". In order to avoid such effects, known techniques have in practice been limited to the use of engraving frequencies of less than 10 kHz. A typical cell density is 22000/cm.sup.2 and a typical roller size is greater than 10000 cm.sup.2 At a rate of 10 kHz, such a roller takes at least six hours to engrave. Significant advantages would be realised by any system making possible an increase in the engraving frequency.